When I want to start a project, I just go through my unorganized stash and pull out any fabrics I think might work. Once I have that pile, I go though and eliminate fabrics. That smaller pile of fabrics becomes my project pile, at which point I could take swatches with me to the store if I think I need something else. However, usually what I end up needing to buy is just border fabric and/or backing fabric and/or binding fabric -- all of which do not need to be purchased until the top is basically finished; I can take the top to the store with me to get exactly the right fabric for the borders, etc.
I have a large stash and the hours I would need to organize it perfectly, and then the additional hours I would need to keep it organized after project "pulls" just isn't worth it to me. I just roughly box my fabrics according to style (batiks together) or color or light fabrics or dark fabrics or long lengths suitable for backings or fat quarters or Oriental fabrics, and leave it at that.